The concept of Activation

All callin bindings of a team only have effect if
the team is active. Activation may be caused by explicit
statements and also happens implicitly at certain points
during program execution.

Guard predicates

Callin bindings can further be controlled using guard predicates,
which can be attached to roles and teams at different levels. If a guard
predicate evaluates to false, all affected callin bindings
are disabled.

Activating a team instance has the effect of enabling all its callin bindings.
All effects defined in §4 apply only if a corresponding
team instance is active.
The order of team activation controls the order of callin executions.
If more than one team intercepts calls to the same base method, the most recently activated
team has highest priority in that its before or replace callins are executed first while its after
callins are executed last.

(a) Activation block

In these statements, myTeam must denote a team instance.
For the time of executing this block, this team instance is activated for the current thread,
which has entered the within block.

The within block statement guarantees that it leaves
the team in exactly the same activation state as it was in when
entering this block. This includes the cases of exceptions, meaning
that deactivation will also occur if the execution of the block
terminates abnormally.

(b) Imperative activation

Each team class inherits methods from the predefined class
org.objectteams.Team (super class of all team classes) to control team
activation disregarding the block structure of the program. The methods activate()
and deactivate() are used to activate and deactivate a team instance for
the current thread.
If a team should be de-/activated for another thread this can be done by the methods
activate(Thread aThread) and deactivate(Thread aThread).
In order to achieve global activation for all threads the predefined constant
Team.ALL_THREADS is passed to these methods (e.g.
activate(Team.ALL_THREADS)).
Note, that this methods make no guarantees with respect to exceptions.

(c) Multiple and mixed activations

If activate() is invoked on a team instance
that has been explicitly activated before, this statement has
no effect at all (note the difference in §5.3.(a) below).
The same applies to deactivating an inactive team.

If a team was already active when entering a within
block, it will remain active after leaving the block.

If the team was active on entry of a within block
and if deactivate() is invoked on the same team
instance from within the within block,
leaving the block will re-activate the team.

When the control flow is passed to a team or one of its roles, the
team is implicitly activated for the current thread. So a programmer may in general assume,
that whenever a role forwards calls to its base object via callout,
the callin bindings of the same role will be active at that time.
Exceptions to this rule have to be programmed explicitly.

(a) Team level methods

While executing a team level method, the target team
is always active. Activation is reset to the previous state
when leaving the team method, unless the team has been explicitly activated during
execution of the team method by a call to activate().
Explicit activation is stronger than implicit activation and thus persists after the
team level method terminates. Ie., leaving a team level method will never reset
an explicit activation.

(b) Methods of externalized roles

Invoking a method on an externalized role
(see §1.2.2) also has the
effect of temporary activation of the team containing the role for the current thread.
Regarding deactivation the rule of §5.3.(a) above applies accordingly.

(c) Nested teams

Implicit activation of a team causes the activation of its outer teams.

Implicit deactivation of a team causes the deactivation of its inner teams.

Note that among the different mechanisms for activation, within is strongest,
followed by (de)activate(), weakest is implicit activation. In this sense,
explicit imperative (de)activation may override the block structure of implicit activation
(by explicit activation within a team level method), but not that of a within
block (by deactivation from a within block).

The effect of callins can further be controlled using so called guard predicates.
Guards appear at four different levels:

callin method binding

role method

role class

team class

Guards can be specified as regular guards or base guards,
which affects the exact point in the control flow, where the guard will be evaluated.

(a) General syntax for guards

A guard is declared using the keyword when followed by a
boolean expression in parentheses:

when (predicateExpression)

Depending on the kind of guard different objects are in scope using
special identifiers like this, base.
Any predicate expression that evaluates to true enables
the callin binding(s) to which it applies.
Evaluation to false disables the callin binding(s).

(b) No side effects

A guard predicate should have no side effects.
A compiler should optionally check this condition, but inter-procedural analysis
actually depends on the availability of appropriate means to mark any method as side-effect free.

(c) Exceptions

A guard predicate should not throw any exceptions.
Yet, any exception thrown within a guard predicate cause the guard to evaluate to false
rather than propagating the exception, meaning that the evaluation of a guard predicate will never
interrupt the current base behaviour.
A compiler should flag any checked exception that is thrown within a guard.
Such diagnosis should by default be treated as an error, with the option of configuring
its severity to warning or ignore.

(a) Method binding guards

Such a guard only affects the callin binding to which it is attached,
i.e., this specific callin binding is only effective,
if the predicate evaluates to true.
The following values are within the scope of the predicate expression,
and thus can be used to express the condition:

The role instance denoted by this.
Features of the role instance can also be accessed relative to
this with or without explicit qualifying this.

The team instance denoted by a qualified this reference as in
MyTeam.this.

If the callin binding includes signatures (as in the example above):
Parameters of the role method.
If parameter mappings are involved, they will be evaluated before evaluating the guard.
Furthermore, if the modifier of the callin binding is after: The result
(denoted by the special identifier result) of the role method, if it is
not void.

(b) Method guards

A method guard is similar to a method binding guard, but it applies
to all callin method bindings of this method.
A method guard is declared between the method signature and the method body:

The following values are within the scope of the predicate expression:

The role instance denoted by this (explicit or implicit, see above).
Thus, in the example value will be interpreted as a field of the enclosing role.

The team instance denoted by a qualified this reference as in MyTeam.this

(d) Team level guards

A guard specified in the header of a team class may disable the callin
bindings of all contained role classes. The syntax corresponds to the syntax
of role level guards.
The only value directly available within team level guard is the
team instance (denoted by this) and its features.

Of course all guards can also access any visible static feature of a visible class.

Even if a guard has no direct effect, because, e.g., a role class has no callin
bindings (maybe not even a role-base binding), predicates at such abstract
levels are useful, because all predicates are inherited by all sub classes
(explicit and implicit).

The intention behind base guards is to prevent lifting of a callin-target
if a guard evaluates to false and thus refuses to invoke the
callin bound role method. Using base guards it is easier to prevent any
side-effects caused by a callin binding, because lifting could cause side-effects
at two levels:

Creating a role on-demand already is a side-effect (observable e.g.
by the reflective function hasRole (§6.1))

However, different scoping rules apply for the identifiers
that can be used in a base guard:

(a) Base object reference

In all base guard predicates the special identifier base
can be used to denote the base object that is about to be lifted.

(b) Method binding guards

A base method binding guard may access parameters as passed to the
base method. Parameter mappings are not considered.
Additionally, for after callin bindings, the identifier result
may be used to refer to the result of the base method (if any).

Note:

In order to achieve the same effect of accessing
the base method's result, a regular binding guard (not a base guard)
must use a suitable parameter mapping (see §4.4.(c)).

(c) Method guards

In contrast to regular method guards, a base guard attached to a role method
cannot access any method parameters. See the next item (d) for values that are actually in scope.

(d) Role level guards

Role level base guards may use these values:

The base instance using the special identifier base.

The team instance using a qualified this references (MyTeam.this).

(e) Team level guards

Team level base guards have the same scope as role level base guards (d).
However, the type of the role instance is not known here, i.e., here base
has the static type java.lang.Object.

(f) Unbound roles

In contrast to regular guards, base guards cannot be attached to
unbound role classes nor to their methods.
Only team level base guards are independent of role binding.

Due to the different ranges of applicability different guards may affect the same method binding.
In that case all applicable guards are conjoined using a logical and.
Any guard is interpreted as the conjunction of these predicates (if present):

The direct predicate expression of the guard.

The next outer guard along the chain
method binding -> method -> role level -> team level

The guard at the same level that is inherited from the implicit super role.

The guard at the same level that is inherited from the explicit super role.

Example code (Guard Predicates):

1

publicteamclass ATM {

2

private Bank myBank;

3

publicclass ForeignAccount playedBy Account

4

base when (!ATM.this.myBank.equals(base.getBank()))

5

{

6

callinvoid debitWithFee(int amount) {

7

base.debitWithFee(fee+amount);

8

}

9

void debitWithFee(int i) <-replacevoid debit(int amount)

10

base when (amount < 1000);

11

}

12

}

Effects:

The team in this example causes that an additional fee has to be payed while debiting
less than 1000 Euros from a "foreign" account.

The base guard in line 4 ensures that Account objects only get
ForeignAccount roles, if they belong to a different bank than the
surrounding ATM team.
It accesses the bank of the base via the base identifier.

The method binding guard in line 10 restricts the callin to
debitWithFee to calls where the base method argument
amount is lower than 1000.

A call to Account.debit causes a replace callin to
debitWithFee only if both predicates evaluate to true.

If an application should be adapted unanticipatedly by one or more teams,
this can be achieved without explicitly changing the program code of this application.

General activation via config file:
Instead of adding the team initialization
and activation code to the main program, it is possible to add the respective teams via a config file.
Every line of this text file contains the fully qualified name of a compiled team, which has to be available
on the classpath.
For the instantiation of these teams the default constructor is used, which means adding a team to
an application this way requires the team to provide a default constructor.
The activation order (see §5.1) for these teams corresponds to the order
in which they are listed in the config file.
Lines starting with a '#' denote comment lines.

Example config file:

To get this config file recognized by the application the VM argument'-Dot.teamconfig=<config_file_name>'
has to be used when starting the application.

Note:

In the ObjectTeams Development Tooling (OTDT) teams are activated unanticipatedly
via a special tab in the "Run-Configuration" (see OTDT features), instead.

Activation adjustment example:
Teams added via the config file mechanism are activated by default. Because no reference to them is
stored anywhere, it is not possible to deactivate them later.
If deactivation of unanticipated added teams is required, this can be achieved by adding a manager team
via config file and encapsulate the actual functionality in another team managed by the manager team.
This way a functional team can be activated and deactivated as needed.

Example code (Activation Adjustment):

1

publicteamclass MyManagerTeam {

2

private FunctionalTeam myFunctionalTeam = new FunctionalTeam();

3

protectedclass MyRole playedBy MyApplication {

4

void startAdaption() { myFunctionalTeam.activate(); }

5

startAdaption <-before startMethod;

6

void stopAdaption() { myFunctionalTeam.deactivate(); }

7

stopAdaption <-after stopMethod;

8

}

9

}

# Config file for the manager team example:MyManagerTeam

Effects:

startMethod and stopMethod are methods which demand
the activation and deactivation respectively.

If the activation/deactivation depends on other conditions these can be checked in addition.